1. Field of the Invention
This invention relates generally to chronically implanted medical leads and, in particular, to a substrate which provides increased dimensional consistency and impedance as well as simplifying manufacture of a sintered porous platinized steroid eluting electrode used in a pacing cardiac lead.
2. Description of the Prior Art
The safety, efficacy and longevity of an implanted pacemaker system depends, in part, on the performance of the pacing leads, the electronic circuits and the integrity of the pulse generator, and the capacity and reliability of the pulse generator power source. These inter-related components of the implanted pacemaker system optimally are matched in a fashion that accommodates ever increasing demands on the modes of operation and function of the system in conjunction with an overall reduction in system size, an increase in system longevity and an increased expectation in system reliability. During the past thirty years, the technology of cardiac pacing has significantly advanced. Implantable pacing systems offer ever increasing variety of pacing modalities, thereby substantially broadening the indications for pacemaker use. In conjunction with this advancement, there has been extensive research and development expended to optimize the performance of pacing leads and their reliability while concurrently simplifying their manufacture.
In the past ten years, substantial improvements in reliable stable chronic pacemaker sensing and stimulation thresholds have been achieved which in turn have allowed the development of smaller and longer-lived pacemakers that can be used with leads having excellent safety margins and reliability. As new circuits are developed with lower "overhead" current drains, however, and as the circuits increase in complexity to allow for ever increasing pacemaker capabilities in their programmable functions, modes and memory, pacemaker longevity depends increasingly more on the characteristics of the lead. In addition, many doctors who implant pacemakers prefer that pacing lead bodies be made ever thinner, to occupy less space in the venous system, without diminishing or detracting from the mechanical strength and integrity of the lead body.
In the early days of cardiac pacing, pacing leads having relatively very high geometric surface area electrodes were employed with relatively bulky and short-lived pulse generators. Early investigators, including Dr. Victor Parsonnet, advanced designs of pacing lead electrodes for achievement of low polarization and low thresholds while presenting a relatively small effective surface area for the delivery of a stimulating impulse in designs known as a differential current density (DCD) electrode of the type shown in U.S. Pat. No. 3,476,116. The DCD electrode (like all pacing electrodes of the time) suffered unacceptable chronic tissue inflammation and instability in clinical testing and was not commercialized.
Subsequent researchers, including Dr. Werner Irnich, explored in considerable detail the electrode-tissue interface and sought to arrive at an optimum exposed electrode surface area for both stimulation thresholds and sensing. In his work "Considerations in Electrode Design for Permanent Pacing" published in Cardiac Pacing; Proceedings of the Fourth International Symposium of Cardiac Pacing (H. J. Thalen, Ed.) 1973, pp. 268-274, Dr. Irnich argued that the field strength (E) required to stimulate cardiac tissue varies according to the following equation: ##EQU1## where v equals applied voltage (threshold, v), r equals electrode radius and d equals fibrous capsule thickness. Dr. Irnich further argued the mean value for d equals about 0.7 mm, regardless of electrode radius. Therefore, the smaller the electrode radius, the lower the threshold (assuming E is a constant) until r equals d. When r&lt;d, thresholds rise again. Dr. Irnich concluded the exposed hemispherical electrode at the tip of the lead should have a radius in the order of 0.7 to 1.0 mm which would result in an exposed surface area of 3-6 mm.sup.2. However, Dr. Irnich went on in his article to propose a design employing wire hooks designed to penetrate the myocardium to hold the electrode in position. These active fixation wire hook electrodes never achieved popularity and were supplanted by passive fixation tined and active fixation screw-in endocardial pacing leads.
In a later paper, "Acute Voltage, Charge and Energy Thresholds as Functions of Electrode Size for Electrical Stimulation of the Canine Heart", by F. W. Lindemarts and A. N. E. Zimmerman; Cardiovascular Research, Vol. XIII, No. 7, pp. 383-391, July 1979, the authors demonstrated an electrode radius of about 0.5 mm is optimal in the acute situation. However, it was recognized the benefits of a small electrode surface area would be lost when the fibrous capsule gets thicker than about 0.5 mm (as Dr. Irnich also stated.) For that reason electrodes of such small surface area, the authors concluded, could not be used chronically.
Dr. Seymour Furman also studied the relationship of electrode size and efficiency of cardiac stimulation and presented a-ball-tip/exposed spaced coil electrode and a small hemispheric electrode in his article entitled "Decreasing Electrode Size and Increasing Efficiency of Cardiac Stimulation" in Journal of Surgical Research, Vol. 11, No. 3, March 1971, pp. 105-110. Dr. Furman concluded the practical lower limit of electrode surface area was in the range of 8 mm.sup.2, observing that impedance increased as an inverse function of the surface area.
Electrodes of many shapes including cylindrical, ball-tip, corkscrew, ring tip and open cage or "bird cage" configurations were pursued with exposed electrode surface areas tending toward 8 mm.sup.2 in the mid-1970's.
More recently, various investigators have emphasized materials and their relationship to the considerations involved in optimizing electrode design. For example, Bornzin, U.S. Pat. No. 4,502,492 owned by Medtronic, Inc. discloses a low polarization, low threshold electrode design which was commercialized as the TARGET TIP.RTM. lead during the early to mid-1980's. That design featured a generally hemispherical electrode with circular grooves, fabricated from platinum and coated over its external surface with a plating of platinum black. This combination of the relatively low (8 mm.sup.2) macroscopic electrode surface area and relatively high microscopic electrode surface area (due to the use of platinum black-) contributed to the achievement of state-of-the-art thresholds for that time period. Other manufacturers marketed electrodes of other materials and configurations including totally porous platinum mesh (Cardiac Pacemakers, Inc.), porous surface sintered (Cordis Corporation), glassy and vitreous carbons (Siemens Inc.), and laser drilled metal (Telectronics Ltd.) electrodes in that same time period.
A considerable breakthrough in the development of low threshold electrode technology occurred with the invention of the steroid eluting porous pacing electrode of Stokes, U.S. Pat. No. 4,506,680 and related Medtronic U.S. Pat. Nos. 4,577,642; 4,606,118 and 4,711,251, all incorporated herein by reference. The electrode disclosed in the Stokes '680 patent was constructed of porous, sintered platinum or titanium, although carbon and ceramic compositions were also mentioned. Proximate the electrode a plug of silicone rubber impregnated with the sodium salt of dexamethasone phosphate, or a water soluble form of other glucocorticosteroids, was placed. The silicone rubber plug allowed the release of the steroid through the interstitial gaps in the porous sintered metal electrode to reach the electrode-tissue interface and prevent or reduce inflammation, irritability and subsequent excessive fibrosis of the tissue adjacent to the electrode itself. The porous steroid eluting electrode presented a sensing (source) impedance substantially lower compared to similarly sized solid electrodes and presented significantly lower peak and chronic pacing thresholds than similarly sized solid or porous electrodes. These two advantages of the steroid eluting electrode allowed a relatively small surface area electrode of about 5.5 mm.sup.2 (CAPSURE.RTM. SP Model 5023, 5523 leads sold by Medtronic, Inc.) to raise lead impedance without sacrificing the ability to sense heart activity. The smaller electrode size was important because it resulted in higher current density during stimulation pulses. This, in turn, was important because it provided more efficient stimulation of the heart tissue with lower current drain from the implanted pacemaker power source. This resulted in overall increased longevity of the implanted pacemaker system.
Lead impedance is a function of the resistance of the lead conductor and the stimulating electrode as well as the effective impedance of the electrode-tissue interface. An inefficient way or means to raise impedance is to increase the resistance of the lead conductor. This wastes current as heat. It is preferable to decrease lead current drain with more efficient control of the stimulating electrode-tissue interface impedance. This can be done by reducing the geometric surface area of the electrode. It has been widely believed that small electrodes, however, are inefficient at sensing natural depolarizations of the cardiac tissue. This is not necessarily true. The amplitude of the intrinsic cardiac depolarization signals typically the ventricular QRS and/or atrial P-wave complexes) is essentially independent of electrode size. The problem of sensing natural depolarizations therefore is that the sense amplifiers of some pulse generators have comparatively lower input impedance, some as low as 20 k.OMEGA., with typical values of about 80-100 k.OMEGA.. The impedance of the QRS or P-wave signal (or "source impedance") increases as the electrode surface area decreases. Thus, a 5 mm.sup.2 polished electrode will produce QRS or P-waves with about 2.5-5 k.OMEGA. source impedance (depending on the material). According to Kirchoff's law, the attenuation of the signal in the generator's amplifier is 1/(1+Z.sub.in /Z.sub.s) where Z.sub.in is the input impedance of the amplifier and Z.sub.s, is the source impedance of the signal to be sensed. Thus, for a signal with a 5 k.OMEGA. source impedance into an amplifier with a 20 k.OMEGA. input impedance will have its amplitude reduced by [1/ (1+20/5)].times.100=20%. In marginal cases, this may make the difference between being able or not being able to sense properly. Therefore, it is important to keep the source impedance low, preferable to attenuate less than 10% of the cardiac input signal, that is, Z.sub.s &lt;2222.OMEGA., for a 20 k.OMEGA. amplifier.
Thus, there is a trade-off with geometric surface area of the electrode between the demands for low current drain and adequate sensing. In addition, it is desirable to achieve relatively low polarization effects to avoid distortion of the electrogram of evoked or intrinsic cardiac depolarizations or leave a postpulse potential of sufficient magnitude to be mistakenly sensed as a QRS or P-wave by the amplifier.
Recent advances in lead design have continued decreasing the exposed geometric surface area of the electrode. One such example is disclosed in U.S. patent application Ser. No. 07/887,560 filed May 18, 1992, now U.S. Pat. No. 5,282,844 and entitled "High Impedance, Low Polarization, Low Threshold Miniature Steroid Eluting Pacing Lead Electrodes" incorporated herein by reference. That application discloses a lead featuring a porous platinized steroid eluting electrode exhibiting an effective surface area in the range of 0.1 to 4.0 mm.sup.2, and preferably between 0.6 to 3.0 mm.sup.2. Such small electrodes, while having many desirable qualities, are relatively more difficult to consistently manufacture, especially with respect to electrode diameter and concentric dimension.
Porous platinized steroid eluting electrodes have been manufactured using a slurry process. Specifically a slurry of platinum particles suspended in a liquid organic binding agent is created. To form the electrode, a substrate, typically a straight shank of wire, is dipped at one end into the slurry. A portion of the platinum mixture sticks to the wire substrate when it is removed. The substrate is then sintered to drive off the binder and fuse the platinum particles together. Under conditions of mass production the resultant electrode is many times, however, dimensionally inconsistent, e.g. eccentric, too large or too small and thus unacceptable.